1. Field of the Invention
The present invention relates to a molded plastic part. In addition, the present invention also relates to a method for molding a plastic part, and to an optical scanning device using the molded plastic part.
2. Description of the Related Art
Conventionally, injection molding methods for use in molding a plastic part, which include injecting a raw material of resin or a melted resin into a certain-volume cavity of an openable die heated to a thermally-deformable temperature of the resin, gradually cooling the resin while controlling the pressure on the resin, and then opening the die to obtain a molded part, are known.
By using such an injection molding method, a plastic part having a complex form can be mass-produced at a low cost as long as a die suitable for producing the part is used. However, since optical elements such as lenses and prisms are required to have high precision in shape of the optical surface thereof and not to induce birefringence in the inner portion thereof, glass has been typically used for such optical elements. According to a recent need for low-cost optical elements (such as lenses and mirrors), the constituent material of such optical parts has been changed from glass to plastics.
There are a variety of molded plastic parts with respect to shape. For example, there is a molded plastic part having a thin portion which is formed by contacting a transfer surface of a die and whose surface has fine convex and concave portions. When such a plastic part is prepared using a die, the die is required to have a high-precision transferability. Particularly, elements (such as lenses) used for optical scanning systems of image forming apparatus such as laser printers have to carry out multiple functions to minimize the number of elements (lenses). Therefore, such elements typically have complex non-spherical mirror surfaces as well as spherical mirror surfaces. In addition, such parts typically have thin-walled shapes to meet space-saving requirements therefor.
When molding such a thin-walled plastic part using a die, it is preferable to maintain the pressure on the resin injected into the cavity of the die and the temperature of the resin so as to be constant in the cooling process in which the melted resin in the cavity is cooled to solidify, in order to prepare a molded plastic part with high dimensional precision. However, in a thin portion of such a plastic part as illustrated in FIG. 1, which has an aspect ratio (i.e., a ratio (a)/(b) of a thickness (a) of the thin portion having a cross section 14 in a first direction perpendicular to a transferred surface of the plastic part to a thickness (b) thereof in a direction parallel to the transferred surface in FIG. 1) of less than 1, the cooling speed in the first direction is faster than that in the second direction, thereby cooling and solidifying the molded plastic part while keeping the pressure on the resin at the transfer wall surface of the cavity (die), resulting in occurrence of problems in that, due to internal stress of the solidifying resin, the molded plastic part is deficiently released from the die (for example, part of the molded part is adhered to the transfer wall surface of the cavity), and the molded plastic part is deformed (bent) after released from the die. Particularly, plastic parts prepared by such a molding method tend to induce a birefringence phenomenon due to internal strain. Hereinafter, a molded plastic part having a thin portion having an aspect ratio (a)/(b) of less than 1 is referred to as a thin-walled plastic parts.
In a case of such a thin-walled plastic part as illustrated in FIG. 4, which has fine convex and concave portions on a transferred surface thereof, it is necessary to increase the pressure in injecting a resin into the cavity so that the injected resin can be satisfactorily contacted with the transfer wall surface of the cavity for forming the convex and concave portions on the plastic part. When the injected resin is cooled, large internal stress (residual pressure) remains in the molded plastic part, resulting in deformation of the molded plastic part and/or occurrence of a birefringence phenomenon when the plastic part is used as an optical element). In this regard, in order to reduce the residual pressure, the injection molding has to be performed while decreasing the injection pressure. However, when low-pressure injection molding is performed, the volume of the injected resin is decreased so as to be relative small compared to the volume of the cavity, thereby forming a sink on a transferred surface of the resultant molded plastic part after the resin solidifies, resulting in deterioration of transfer precision (i.e., preparation of a part having an undesired shape due to volume contraction).
In attempting to solve the sink formation problem in low-pressure injection molding, there is a proposal for a die in which air is applied from an air hole to a non-transferred surface of a resin to generate pressure difference between a transferred surface and the non-transferred surface so that a sink is formed on the non-transferred surface while preventing formation of a sink on the transferred surface.
In addition, there is a proposal for a molding method in which at least one of cavity pieces forming a cavity is slid at a proper time in a molding operation to separate a non-transferred surface of the resin therefrom, i.e., to form an air gap therebetween, so that a sink is formed on the non-transferred surface while preventing formation of a sink on the transferred surface.
Further, there is a proposal for a molding method for molding a thick part or a part having uneven thickness, in which the injected resin is subjected to imperfect transfer at a surface of a cavity of a die other than a transfer surface thereof to forcibly form a sink on the imperfectly transferred surface while preventing formation of a sink on a transferred surface.
The above-mentioned techniques are not suitable for molding thin-walled plastic parts.
In addition, recently, the requirements for such molded plastic parts (such as plastic optical elements) become severe and severe to produce higher-quality optical images using such molded plastic parts.
For these reasons, the inventors recognized that there is a need for a molded thin-walled plastic part which can exhibit good optical property when used for optical scanning systems without causing the sink formation problem and the birefringence phenomenon.